The invention relates to a picture display device having a vacuum envelope which is provided with a transparent face plate and a display screen having a pattern of luminescent pixels, and with a rear wall, comprising electron-producing means, an addressing system arranged between said means and the face plate so as to address desired pixels, and, adjacent to the display screen, a plate of electrically insulating material provided with apertures for passing electrons.
The display device described above is of the thin-panel type. Display devices of the thin-panel type are devices having a transparent face plate and, arranged at a small distance therefrom, a rear plate, while a (for example, hexagonal) pattern of phosphor dots is provided on the inner surface of a face plate. If (video information-controlled) electrons impinge upon the luminescent screen, a visual image is formed which is visible via the front side of the face plate. The face plate may be flat or, if desired, curved (for example spherical or cylindrical).
A thin-panel display device described in U.S. Pat. No. 5,313,136 (=PHN 12.927) comprises a plurality of juxtaposed sources for emitting electrons, local electron propagation means cooperating with the sources, each having a wall of a high-ohmic, electrically substantially insulating material having a secondary emission coefficient which is suitable for propagating emitted electrons, and an addressing system comprising electrodes (selection electrodes) which can be driven row by row so as to extract electrons from the propagation means at predetermined extraction locations facing the luminescent screen, while further means are provided for directing extracted electrons towards pixels of the luminescent screen for producing a picture composed of pixels.
Other display devices of the thin-panel type to which the invention pertains are, for example, plasma displays and, in particular, field emission displays.
The luminescent screen is also referred to as the phosphor screen. An important component of the above-mentioned display device is an apertured plate of electrically insulating material, in many applications described as "the screen spacer".
The screen spacer is adjacent to the phosphor screen. Due to the efficiency and the saturation behaviour of the phosphor, it is of crucial importance that the acceleration voltage to the phosphor screen is as high as possible. Dependent on the phosphors used, 3 kV or, more frequently, 4 to 5 kV is a minimum requirement.
The screen spacer is made of an insulating material, particularly glass. The face plate is provided with a low-ohmic transparent conducting electrode of, for example ITO. This coating is provided with the phosphor screen and (possibly) a black matrix. A typical thickness of the screen spacer is 0.3 or 0.4 to 1.0 mm. The voltage difference between the input side of the screen spacer and the ITO coating should be as high as possible. At large voltage differences a number of unwanted effects in the form of picture errors may occur. The invention is based on the recognition that these effects are related to the "vacuum current" flowing through the screen spacer. The invention provides a display device of the type described in the opening paragraph, having surfaces, particularly at the electron entrance side of the screen spacer, treated in such a way that the occurrence of these unwanted effects (which, according to the invention, are based on secondary emission of electrons backscattered from the display screen at voltage differences of at least 5 kV across the spacer) are obviated entirely or partly. For this purpose, a coating is preferably used which has such a composition that its properties are stable under electron bombardment. This contributes to the lifetime.
To this end, an embodiment of a display device of the type described in the opening paragraph is characterized in that the surface at the entrance side of the apertured plate is coated with a coating of a material selected from the group comprising nonstoichiometric nitrides, borides and carbides of Al and/or Si, and amorphous Si, optionally doped with N and/or H.
It has been found that with the above coatings, which were found to be stable under electron bombardment, electrical resistances between 10.sup.10 .OMEGA./.quadrature. and 10.sup.14 .OMEGA./.quadrature. can be realized, which values are eminently suitable for the purpose of the invention, values between 10.sup.10 .OMEGA./.quadrature. and 10.sup.13 .OMEGA./.quadrature., and in particular 10.sup.11 to 10.sup.12 .OMEGA./.quadrature., being preferred. Of the above materials SiN.sub.x (0&lt;x.ltoreq.1.3) is very well suited for an industrial process. Suitable resistance values are obtained in particular if 0&lt;x.ltoreq.0.4.
It appears that at higher acceleration voltages, the unwanted effects leading to picture errors can effectively be prevented when these coatings are used. Further the resistance of the material of the apertured plate should be sufficiently high. This resistance R (in .OMEGA. cm) preferably satisfies logR.gtoreq.12.
The required coatings may be provided by means of plasma CVD or, preferably, (rf or dc) magnetron sputtering. Generally, the surface of the plate and the walls of the apertures are coated therewith, while leaving the choice of coating at one or two sides.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.